INVESTIGATION OF DENSITY-DEPENDENT GAS ADVECTION OF TRICHLOROETHYLENE- EXPERIMENT AND A MODEL VALIDATION EXERCISE

An experiment was conducted to evaluate whether vapor-density effects are significant in transporting volatile organic compounds (VOC's) wit h high vapor pressure and molecular mass through the subsurface. Trich loroethylene (TCE) was chosen far the investigation because it is a co mmon VOC contaminant with high vapor pressure and molecular mass. For the investigation, a 2-m-long by 1-m-high by 7.5-cm-thick flow cell wa s constructed with a network of sampling ports. The flow cell was pack ed with sand, and a water table was established near the lower boundar y. Liquid TCE was placed near the upper boundary of the flow cell in a chamber from which vapors could enter and migrate through the sand. T CE concentrations in the gas phase were measured by extracting 25-mu l gas samples with an air-tight syringe and analyzing them with a gas c hromatograph. The evolution of the TCE gas plume in the sand was inves tigated by examining plots of TCE concentrations over the domain for s pecific times and for particular locations as a function of time. To h elp in this analysis, a numerical model was developed that can predict the simultaneous movements of a gas, a nonaqueous liquid and water in porous media. The model also considers interphase mass transfer by em ploying the phase equilibrium assumption. The model was tested with on e- and two-dimensional analytical solutions of fluid flow before it wa s used to simulate the experiment. Comparisons between experimental da ta and simulation results when vapor-density effects are considered we re very good. When vapor-density effects were ignored, agreement was p oor. These analyses suggest that vapor-density effects should be consi dered and that density-driven vapor advection may be an important mech anism for moving VOC's with high vapor pressures and molecular mass th rough the subsurface.